This invention relates to an engine exhaust system designed to reduce hydrocarbon emissions therefrom. More specifically, this invention is concerned with overcoming pollution problems associated with engine start-up, when, because traditional catalytic converter systems have not yet reached an efficient operating temperature, hydrocarbon gases are discharged by the engine exhaust system in great amounts.
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline, or diesel fuel, can cause serious pollution of the atmosphere. Among the pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NO.sub.x) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of gaseous emissions from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in model year 1975. The catalytic converters generally utilize noble metal catalysts capable of converting hydrocarbons, CO, and NO.sub.x to non-toxic products water, carbon dioxide, and reduced nitrogen species.
The catalysts utilized in catalytic converter systems are generally inefficient or inactive at ambient temperature and must reach high temperatures, often in the range of 300-400.degree. C., before they are activated. Typically, the temperature of the catalyst is elevated by contacting it with the high-temperature exhaust gases from the engine. Continuous contact with those gases and the exothermic nature of the oxidation reactions occurring at the catalyst combine to maintain the catalyst at an elevated temperature. The temperature at which a catalytic converter can convert 50% of carbon monoxide, hydrocarbons, or NO.sub.x is referred to as the "light-off" temperature of the converter.
During start-up of current commercial engines, the amounts of carbon monoxide and hydrocarbons in the exhaust gas are higher than during normal engine operation. For example, as noted in U.S. Pat. No. 3,896,616, the amount of carbon monoxide at start-up may be on the order of about 3 to 10 or more percent by volume (versus about 0.5 to 3% CO during normal operation), and the amount of hydrocarbons can typically be about 750 to 2,000 parts per million (ppm) (versus about 100 to 750 ppm during normal operation). Applicants' experiments have detected hydrocarbon emissions that are significantly higher even than these reported levels, particularly those generated during start-up. Thus, a large portion of the total emission generated by an internal combustion engine is generated in the first few minutes of operation. Unfortunately, at start-up, when the catalytic converter is most needed, it may be relatively ineffective because it will not have reached a temperature at which it is activated.
There have been numerous suggestions for avoiding the pollution problems inherent in engine start-up, as noted by U.S. Pat. No. 3,896,616. For example, it has been suggested to electrically heat the catalytic converter before starting the engine, but this would unduly increase costs and also cause inconvenient delays before the engine could be started cleanly. It has also been recommended that the catalytic converters be placed as close to the engine as physically possible to minimize the emission of pollutants during the initial engine start-up. The closer the catalyst is to the engine, the hotter will be the exhaust gas when it contacts the catalyst and the more quickly the temperature of the catalyst will be raised to operating level. However, because of limitations of space in most vehicles, locating the total amount of catalyst in the system near the engine is difficult.
U.S. Pat. No. 3,896,616 suggests that excellent purification of engine exhaust gas is obtained by utilizing an initial catalyst, preferably in a converter vessel placed near the engine, for instance, closely adjacent the exhaust manifold, and a down-stream in-line catalyst. The initial catalyst, being close to the engine, will supposedly reach its effective operating temperature significantly sooner than the in-line catalyst. On cold engine start-up, however, during the period before the initial catalyst reaches its effective temperature, substantial quantities of pollutants would still be introduced to the atmosphere. In addition, because the initial catalyst is positioned close to the engine, it can be overheated, causing degradation and loss of effectiveness.
Accordingly, there remains a need for an engine exhaust system that can reduce the amounts of pollutants introduced into the atmosphere during the critical engine start-up period.